Analysis of Vehicle Speed Measurement Using Magnetic Loops 2001-01-0811

A magnetic loop may be used to compute vehicle speed. In this paper we will analyze magnetic loops similar to those installed on Michigan highways, which are relatively wide and diamond shaped. Speed computation using one loop is dependent on vehicle length, which may vary widely among different cars. This would produce extremely inaccurate results. We analyze another approach that recommends the installation of two loops in succession. The distance between the two loops should be known and the loop controller yields the times the loops turn on. This approach yields vehicle speed as a function of the distance between the loops, the time the controllers turn on, and the inter-arrival time between them and hence it is independent of vehicle length. We analyze the situation where some vehicles may trigger the loops earlier than others because of higher mass concentration of metal in the front of the vehicle, and the situation where the two loops may not be exactly similar.